(a) Field of the Invention
The present invention relates to a display device using liquid crystals, and more particularly, it relates to a display device of a guest-host type using a mixture of nematic liquid crystals having a negative dielectric anisotropy and pleochroic dye.
(b) Description of the Prior Art
There have been proposed, heretofore, many display devices using a guest-host type combination of liquid crystals and pleochroic dyes, which devices in general comprise a cell structure having a mixture of at least one of nematic liquid crystals as host material and at least one of pleochroic dyes as guest material placed in a space between two parallel substrates, as disclosed in, for example, Japanese Patent Publication No. 47-3778 published Feb. 2, 1972.
Such conventional guest-host type display cells are of the so-called negative type, that is, they are colored when no voltage is applied, while they become faded out upon application of a voltage between the electrodes which are formed on the inside surfaces of the substrates. It has been found, however, that the positive-type display cells, i.e. those which are capable of effecting a colored display upon application of a voltage and being faded out when the voltage is removed, are more convenient to use. Although some positive-type display cells using the guest-host type combination of liquid crystals and dyes have been proposed heretofore also, they have many disadvantages. That is, in the cell structure of prior-art such device, the molecules of liquid crystals and dyes are rendered to a very mobile state in any directions immediately after a voltage is applied. Therefore, the molecules will move in random fashion in all directions, and subsequently they will gradually assume a stationary state, with the axes of all these molecules being not oriented in one direction. As a result, known positive-type display cells have a lower rate of response and have a tendency to effect a non-uniform or mottled display.